What can we learn about reconnection from coronal mass ejections?

It may be possible to calculate the rate of reconnection in the corona by m easuring the rate at which the temporary coronal hole formed by a coronal m ass ejection (CME)) disappears. This calculation is possible if the disappe arance of the hole is caused by the same reconnection process which creates the giant X-ray arches associated with CMEs. These arches form just below the vertical current sheet that is created as the CME drags magnetic field lines out into interplanetary space, and they are similar in form to 'post' -flare loops, except that they often have an upward motion that is differen t. Instead of continually slowing with time as 'post'-flare loops do, they move upwards at a rate which increases, or remains nearly constant, with ti me. This difference has raised doubts about the relevance of reconnection t o the formation and propagation of the arches. Using a two-dimensional flux rope model to calculate the size and location of the current sheet as a fu nction of time, we find that the difference between the motion of 'post'-fl are loops and giant arches can be explained simply by the variation of the coronal Alfven speed with height. (C) 2000 Elsevier Science Ltd. All rights reserved.